The use of strained silicon devices is known to increase semiconductor device performance. For example, in the context of transistors, strained silicon increases the transistor drive current which improves switching speed by making current flow more smoothly. Generally, a very thin layer of single-crystal silicon with built in stress (or strain) improves drive current making the devices fun faster. When the layer of silicon is under stress, the silicon lattice lets electrons and holes flow with less resistance. For transistors, the lower resistance translates in to faster switching properties, thereby permitting semiconductor devices to operate at faster speeds.
Because of the advantages inherent in the strained lattice structure, strained silicon or silicon germanium based devices have become an attractive alternative to current microelectronic devices that are composed of a silicon channel layer on a silicon substrate. Several approaches have been developed to form strained silicon on substrates. For example, relaxed silicon germanium buffer layers have been employed as a “virtual substrate” to grow strained silicon. Typically, the relaxed silicon germanium buffer layer, which has a higher lattice constant than the silicon substrate, is formed in a graded manner and is used as an epitaxial growth template.
If a constant (i.e., non-graded composition) silicon germanium buffer layer is used, high densities of dislocations nucleate during growth and interact with one another. This interaction prevents dislocations from propagating to the edge of the substrate (e.g., a wafer), thereby leaving a significant number of threading arms on the surface of the silicon germanium layer. In contrast, by grading the germanium composition during growth of the relaxed silicon germanium layer on a silicon substrate, the nucleation rate of dislocations is retarded by reducing the strain accumulation rate. Consequently, the interaction between dislocations is reduced, significantly reducing the density of threading arm dislocations on the surface of the silicon germanium layer. For example, the threading dislocation density in a constant (non-graded) silicon germanium grown directly on a silicon substrate is on the order of about 108˜9/cm2. If a graded silicon germanium buffer layer is formed on a silicon substrate, the threading dislocation density improves to around 104˜5 cm2.
Unfortunately, there are several disadvantages to graded silicon germanium buffer layers. First, the threading dislocation density, while lower in graded buffer layers, is still non-zero, which leads to degradation of electron and hole mobility. Moreover, a large thickness of graded silicon germanium buffer layer is needed for achieving low threading dislocation densities. The large thickness increases the size of the devices as well as the cost of production. Second, the strain-relaxed graded silicon germanium buffer layer has a rough surface which degrades the mobility of strained silicon. In addition, the strain at the top layer of silicon is not homogeneous due to the stress fields from buried dislocations, which also adversely affects carrier transport.
Attempts have also been made to form relaxed, dislocation free buffer templates of silicon germanium on viscous glass layers. For example, Hobart et al. disclose a process of relaxing compressively strained heteroepitaxial silicon germanium films bonded to a viscous borophosphosilicate glass (BPSG) film. Hobart et al., Compliant Substrates: A Comparative Study of the Relaxation Mechanisms of Strained Films Bonded to High and Low Viscosity Oxides, Journal of Electronic Materials, Vol. 29, No. 7, 2000. In this method, compressed silicon germanium thin film “islands” are transferred to a viscous BPSG layer by wafer bonding. The metastable compressed silicon germanium islands start to undergo elastic relaxation at heating of near 800° C. Because of the island patterning, the small silicon germanium islands may allow faster relaxation than dislocation introduction. Unfortunately, using this method, dislocations will develop if the thickness of the silicon germanium is too large. Typically, the maximum strain in a dislocation-free film that can be produced is around 1% if the film is kept thinner than about 10 nm. On the other hand, if the film is too thin, the relaxation process may introduce wrinkles into the silicon germanium film.